---Do you think we've missed the bus?
--Don't worry. It never exactly comes on the half hour like it should.
Should I replace "on" with "at" since we often say "at 8:30"?
Plus, if a bus comes on the half hour, does it mean it often arrives at 7:30, 8:30, 9:30...?
There are two standard time phrases like this - "on the hour" and "on the half hour". If a bus service is scheduled to stop somewhere twice each hour, on the hour and on the half hour, then it should come at exactly 8.00, 8.30, 9.00, 9.30 etc.
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.